1. Field of the Invention
The present invention relates to a voltage-controlled reactive power controller for controlling a reactive power output from a self-commutated inverter connected to an AC system power source by adjusting an output AC voltage of self-commutated inverter.
2. Description of the Background Art
A device for controlling a reactive power output from a self-commutated inverter has been known, as can be found in Japanese Patent Publication No. 59-44646 by Tada et al. or in Japanese Patent Application No. 62-154343 (corresponding to U.S. patent application Ser. No. 203,140) by one of the present inventors, which controls the reactive power output from the self-commutated inverter by regulating a DC voltage on the self-commutated inverter.
Such a device most typically comprises an inverter including controllable rectifier elements such as gate turn-off thyristors (referred hereafter as GTO) and diodes, a DC capacitor connected in parallel to the inverter, a reactor connected between the inverter and an AC system power source, and means for regulating a DC voltage on the inverter.
Now, such a device is known to have a following problem.
Namely, the DC capacitor is charged up by the AC system power source even when the inverter is not in operation in which case the GTOs of the inverter are not conducting electricity. This is due to the fact that the diodes of the inverter are forming a rectifier circuit. The DC voltage over the DC capacitor in this case is equal to (1.41.times.V.sub.SY) where V.sub.SY is the supply voltage of the AC system power source. When the inverter is put in operation so as to control a reactive power, the reactive power due to the DC voltage (1.41.times.V.sub.SY) over the DC capacitor, which is initially present, cannot be controlled and will affect the AC system power source.
In the prior art reactive power controller, there is provided a control compensator which eventually eliminates the effect of this uncontrolled reactive power. However, the operation of the control compensator to eliminate the effect of the initial uncontrolled reactive power takes a finite amount of time depending on the response speed of the control compensator, so that the adverse effect of the initial uncontrolled reactive power on the AC system power source cannot completely be ignored and remains as an outstanding problem associated with the prior art reactive power controller.